1. Field of the Invention
This invention relates generally to quasi-phase-matched optical frequency conversion and more particularly, first, to methods for intensity modulating frequency converted waves and for optimizing the efficiency of quasi-phase-matched optical frequency conversion; and, second, to the improvement of existing quasi-phase-matched optical frequency converters; the improvement consisting of integral intensity modulation means which both electro-optically frustrate the quasi-phase-matching condition and allow optimization of the quasi-phase-matching condition.
2. Description of the Related Art
Devices which employ quasi-phase-matching to achieve optical frequency conversion are well known in the art. They include frequency doublers, known in the art as second harmonic generation (SHG) devices, and frequency mixing devices. Optical frequency conversion by quasi-phase-matching (QPM) is generally dependent on nonlinear optical effects observed in waveguides, for example, formed by surface modification of preferred inorganic, bulk single crystal materials. Surface modification techniques used to form waveguides in these bulk crystal materials include proton exchange, in-diffusion of metal dopants, and out-diffusion of substrate constituents, all of which are well known in the art.
Alternatively, thin film waveguide frequency converters are under development. Quasi-phase-matched frequency conversion in inorganic nonlinear optical thin film waveguides is taught in a copending application of Agostinelli et al., while Khanarian et al., U.S. Pat. No. 4,865,406 teaches the use of nonlinear organic thin film waveguides to achieve SHG by quasi-phase-matching.
Intensity modulation of the frequency converted output is often desirable or required for the useful application of device output; for example, to write data onto an optical recording medium, to create a photonic data stream for optical communications, to create an image when the modulated light is further scanned in a raster format, and so on. The modulation of frequency converted light may be achieved either by modulating the input (fundamental frequency) light or by modulating the output (frequency converted) light. A wide variety of modulation techniques are known in the art. With respect to commercially important diode laser fundamental light generators, for example, it is convenient to directly modulate the input light by varying the current supplied to the diode laser. Although this method is convenient, it is ill-suited as a means for modulating quasi-phase-matched frequency converted light due to the sensitive relationship between the efficiency of quasi-phase-matched frequency converters and the input optical frequency. It is well known that the optical frequency of semiconductor diode laser output is highly temperature dependent and that the current supplied to the laser directly influences laser temperature. A constant current supply, therefore, is preferred when diode lasers are used as input sources for frequency converters employing quasi-phase-matching, and to modulate the frequency converted output by other means.
External modulation of frequency converted light is very well known in the art. Known means involve devices which use the electro-optic or acousto-optic effect, and these modulators may be discrete or integrated. Discrete devices include Pockels cells, acousto-optic deflectors and electro-optic deflectors; while Mach-Zehnder interferometers, electro-optic directional couplers, Bragg deflectors and total internal reflection waveguide crosses are examples of integrated devices. Although some of these techniques are compatible with quasi-phase-matching nonlinear optical frequency converter devices, their use considerably increases overall device complexity. In addition, these modulation techniques lack the degree of dynamic optimization of the efficiency of quasi-phase-matched frequency conversion offered by the the present invention.
Direct modulation of the second harmonic output from birefringently phase-matched bulk crystals is also known in the art. These means are inapplicable to quasi-phase-matching waveguide devices, however, since QPM frequency converters do not rely on birefringence to achieve phase-matching.
To summarize, presently utilized means for modulating quasi-phase-matched frequency converted light exhibit shortcomings which limit their ease of use and application utility.